1. Field of the Invention
This invention relates to a solid-state imaging device for use in a television camera etc. Particularly, it relates to a solid-state imaging device which has a plurality of picture elements disposed in a surface region of a semiconductor body. More specifically, it relates to a solid-state imaging device which has picture elements for reading out from photodiodes photo information stored therein.
2. Description of the Prior Art
FIG. 1 is a principle diagram of a prior-art solid-state area sensor. The sensor comprises a photosensitive portion which consists of a large number of photodiodes 1 arrayed in the form of a matrix, vertical switching insulated-gate field effect transistors (hereinbelow, written "MOSFETs") 2 and horizontal switching MOSFETs 3 which serve to read out optical signals stored in the photodiodes 1, and a shift register 4 of a vertical scanning circuit and a shift register 5 of a horizontal scanning circuit which serve to change-over the respective switches in good order. Numeral 6 designates a vertical scanning line, numeral 7 a vertical signal output line, numeral 8 a horizontal scanning line, numeral 9 a horizontal signal output line, numeral 10 an output terminal, numeral 11 an output load resistance, and numeral 12 a voltage source for video outputs. The vertical and horizontal switching MOSFETs effect the switching operations in such a manner that scanning pulses derived from the outputs of the respective stages of the shift registers are impressed on the gates of the MOSFETs.
In this solid-state imaging device, in the photodiode 1 on which intense light with incident photons exceeding a predetermined quantity is impinging, charges to be stored are more than the saturation amount and overflow the vertical signal output line 7, to affect the readout of the other photodiodes 1 connected with the identical vertical signal output line 7. This gives rise to the phenomenon called "blooming" in which white stripe lines appear on a picture screen, so that the picture quality is conspicuously degraded.
As a countermeasure against this drawback, there has been one wherein a MOSFET for drawing excess charges the source of which is connected to the photodiode 1 is disposed and wherein a control line for a bias voltage and an absorptive line for excess charges are respectively connected to the gate and drain of the FET. Since, however, the area of the solid-state imaging device is fixed, the addition of such elements and wirings decreases the area of the photodiodes to that extent, resulting in lowering the sensitivity. Particularly, the gate voltage control lines and the excess charge absorptive lines occupy large areas and drastically degrade the sensitivity.
Further, according to a solid-state imaging device employing an overflow drain which has been developed as a more advanced measure, the photodiode 1 is provided with a MOSFET separately from the switching MOSFETs, and the excess charges are let escape through this MOSFET with a predetermined D.C. bias voltage applied to the drain thereof. Accordingly, an additional absorptive line for excess charges is also required in this device, so that the area of the photodiode decreases to lower the incident photon efficiency and to reduce the saturation charges.